Shure Tri-Polar Microphone Model 720A

FAQ #5014 Updated August 28, 2017

Question:

Did Shure manufactured a tri-polar microphone before the KSM44A?

Answer:

Indeed we did. It was the model 720A, introduced in 1937.  Like the KSM44A, the model 720A was switchable between "Non-Directional" (omnidirectional), "Uni-Directional" (cardioid), and "Bi-Directional" (figure-eight). The retail price was $39.50; in 2016 dollars, that would be $662.25.

The 720A had two crystal elements (one omnidirectional and one bidirectional); same for the KSM44A except the elements are condenser. The 720A crystal elements were employed under a licence from the Brush Development Company: Information about Brush

To create the cardioid pattern, the omnidirectional element is electronically combined with the bidirectional element.

From the 1937 literature: "Here for the first time is one microphone that does everything! The new Shure Tri-Polar Controlled Direction Crystal Microphone gives you all three basic directional characteristics in one microphone, each instantly available though a 3 point switch."

Two technical papers about the model 720A can be found here: Vintage Tech Bulletins

The brilliant Shure microphone engineer, Ben Bauer, wrote the following in his lab notebook, dated August 4 - 7, 1937: "These days were practically completely spent on specifications and claims of tri-polar cosine element patent. Good advancement has been made."

Below are advertising pages and a data sheet for the model 720A.    

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KSM44A

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Vintage Shure Technical Bulletins

Shure and the Brush Company in the 1930s

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